Utility meters are used for billing commodities provided by public utilities such as power, gas, and water. For example, watt-hour meters, located at the customer premises, include meter units for measuring and recording electric power consumption by the customer. Typically, an induction-type watt-hour meter is provided at each customer location. The induction-type watt-hour meter operates on a principle of a rotating magnetic field of an induction motor. Electric power service is routed through the meter in a manner causing a metallic disk to rotate at a rate proportional to power consumption. Disk rotation is counted and recorded mechanically using a mechanical kilowatt hour register and/or electronically with data stored in a conventional semiconductor memory.
In such systems, there is the possibility that a dishonest customer will tamper with the meter to indicate a lower rate of consumption, resulting in an improperly low gas or electric bill. Numerous ingenious methods and devices have been devised by dishonest customers to cheat on such bills. A frequently used method of tamper is to impose a strong static energy field on the meter that is of the same type of energy as the field of energy being counted by the meter. With the aforementioned watt-hour meter, a way to affect the count related to consumption is to impose a magnetic field proximate the meter. It is desirable for such a meter to accurately count the consumption of the commodity and to detect tampering with the meter.
Presently, the most widely used means of counting is performed by using a small magnet embedded at the radius of a rotating shaft which is part of the mechanical function of the meter. A magnetic reed switch positioned near the shaft provides a low power sensor to count the rotation. One of the drawbacks of using a magnetic reed switch in this application is the relatively high level of handling required in volume production of meters. The reed switches typically are two contact metal strips sealed in a slender glass tube that is approximately one half inch in length. This package cannot be easily utilized in auto-place robotic assembly. Additionally, the magnetic reed switches are prone to variations in switching fields which can require additional quality/performance checks to be performed.
In the past, a number of magnetic tamper device, sensors have been used in utility meters. Such sensors have always been placed at a location within the meter that is remote from the count sensor. Detection of magnetic tamper is more accurately sensed in the closest proximxity to the site in the meter that the counting is sensed, rather than at a location that is spatially distant from the site of the count detection. In the past, the selection of a remote location for the tamper sensor has been an engineering compromise. The problem with existing tamper sensors is being able to place the count and tamper sensors in close proximity without the tamper sensor responding to the count actuating magnet. Responses by the tamper sensor to the count actuating magnet result in "false" tamper detections. It is highly undesirable to have a high percentage of "false" tamper detections. Such detections defeat the initial intent of a tamper sensor and result in costly trips for personnel into the field to respond to "false" tamper detections. Placing the tamper sensor at a location far enough from the count sensor location in the meter for the tamper sensor to be relatively immune from "false" tamper detections resulting from sensing the rotating magnetic field typically results in placement of the tamper sensor in a position in which actual tampers may not be detected.
Real world count and tamper sensors have tolerances that must be accounted for. These tolerances arise during the manufacture of such sensors. Accordingly, a production lot of count sensors, the entire lot ostensibly being identically produced, will also have a range of count thresholds. Similarly, a production lot of tamper sensors will have a range of tamper thresholds. The range of possible thresholds for both the count and tamper sensors must be taken into account in design of the sensor. It is virtually impossible with existing reed switch counting sensors to be able to detect all counts and all tamper situations without generating any false tamper detections.
Even solid state sensor devices are not immune form tolerance differences. For example, solid state devices are typically formed in layers on a disc of silicon substrate. The disc may be four to eight inches in diameter. Hundreds of nominally identical solid state devices are formed on the same silicon disc. The individual layers that extend across the full surface of the disc are typically formed by sputtering or similar techniques. The target for the sputtering application is the center of the silicon disc. As the sputtered deposition extends radially outward from the target center, there are variations in the layer that is deposited. These variations in the deposition layer result in tolerance differences between a sensor that was formed close to the target and a sensor that was formed close to the periphery of the disc.
Because of the aforementioned tolerances and the fact that nominally identical sensors are randomly selected for inclusion in a sensor assembly, there virtually always exists a condition where a tamper has occurred and has not been detected or a false tamper has been detected. FIG. 1 depicts this prior art situation. The tolerance variation typically results in the .+-. forty percent that is noted. The output voltage of any given sensor is depicted along the ordinate and the applied field (in this case a magnetic field) that is seen by the sensors is depicted along the abscissa. Selecting sensors at random results in either an overlap in sensor detection by the count and tamper sensors resulting on false tamper detections in the region between H.sub.4 -H.sub.5 or in a gap between the count and tamper thresholds in the region between H.sub.3 -H.sub.4 in which actual tamper conditions will not be detected.
FIG. 2 illustrates the prior art situation in which the influence of the static field causes a certain percentage of sensors in the indicated band to detect neither a count nor a tamper. The graph in the upper portion of FIG. 2 depicts sensor response in a manner that is similar to that of FIG. 1. A gradient field is depicted below the graph depicting a gradient field through 2.pi. or 360.degree. of rotation. The gradient field to the left is without the influence of a static tamper field. The effect of the imposition of the tamper field is to shift the gradient field to the right as indicated in the rightmost gradient field depiction. The minimum point on the gradient field is greater than the maximum count detection threshold H.sub.2 resulting in no count.
In the past, the tolerance variation of the individual sensors used for the count detector and for the tamper detector, though nominally having identical characteristics, resulted in the rejection of many sensors, for their individual tolerances were not compatible for use as count detectors and tamper detectors. This yield rate for sensor assemblies utilizing such randomly selected sensors was unacceptably low.
Magnetometers and other magnetic sensing devices are used extensively in many kinds of systems including magnetic disk memories and magnetic tape storage systems of various kinds. Such devices provide output signals representing the magnetic fields sensed thereby in a variety of situations.
Magnetometers can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic states, or magnetic conditions, therein. Such devices may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the device and the operating circuitry therefor.
In the recent past, reducing the thicknesses of the ferromagnetic thin-films and the intermediate layers in extended "sandwich" structures having additional alternating ones of such films and layers, i.e, superlattices, have been shown to lead to a "giant magnetoresistive ratio effect" being present. This effect yields a magnetoresistive response which can be in the range of up to an order of magnitude greater than that due to the well-known anisotropic magnetoresistive response.
The resistance in the plane of a ferromagnetic thin-film is isotropic with respect to the giant magnetoresistive ratio effect rather than depending on the direction of a sensing current therethrough as for the anisotropic magnetoresistive effect. The giant magnetoresistive ratio effect has a magnetization dependent component of resistance that varies with the angle between magnetizations in the two ferromagnetic thin-films on either side of an intermediate layer. In the giant magnetoresistive ratio effect, the electrical resistance through the "sandwich" or superlattice is lower if the magnetizations in the two separated ferromagnetic thin-films are parallel than it is if these magnetizations are antiparallel, i.e., directed in opposing directions. Further, the anisotropic magnetoresistive effect in very thin-films is considerably reduced from bulk values therefor in thicker films due to surface scattering, whereas very thin-films are a fundamental requirement to obtain a significant giant magnetoresistive ratio effect.
In addition, the giant magnetoresistive ratio effect can be increased by adding further alternate intermediate and ferromagnetic thin-film layers to extend a "sandwich" or superlattice structure. The giant magnetoresistive ratio effect is sometimes called the "spin valve effect" in view of the explanation that a larger fraction of conduction electrons are allowed to move more freely from one ferromagnetic thin-film layer to another if the magnetizations in these layers are parallel than if they are antiparallel with the result that the magnetization states of the layers act as sort of a valve.
There is a need in the industry to provide the following improvements to a meter:
(1) Autoplacement, e.g. robotic placement, of a sensor or sensors onto a circuit board; PA1 (2) Providing count and magnetic tamper detection at substantially the same location within the meter; PA1 (3) Provide an "overlap" in tolerances of the count and tamper detector without setting false tamper alarms; PA1 (4) Provide an "overlap" in tolerances of the count and tamper detector without missing any actual magnetic tamper; PA1 (5) Improve the manufacturing yield of the sensor; and PA1 (6) Provide a gradiometer count sensor that will continue to count until the device reaches saturation.